Do I really need an additional subscription although I have already subscribed to one of your products?
Yes. It is not possible to use the same subscription for both products, Merlin Project Express and Merlin Project Go. Merlin Project Express and Merlin Project Go are two completely different products for different operating systems, macOS and iOS. They therefore require individual subscriptions.
If you have questions regarding subscriptions in the App Store, you will find some answers on the Apple help page.